1. Field of the Invention
The present invention relates to a new and renewable energy generation system, and more particularly, to an anti-islanding protection method of the new and renewable energy generation system.
2. Description of the Related Art
Up to now, various anti-islanding protection methods used in a photovoltaic generation system have been developed. The islanding detection methods may be classified into a passive method and an active method. The Over/Under voltage and frequency detection method, which is the most general passive method, is to stop an inverter when a detected voltage/frequency is deviated from a preset value. While the method is simple, it fails to detect islanding when power generated by the inverter is identical to the capacity of a load connected with a grid.
In the phase jump detection method, the phase of inverter current is instantaneously synchronized with a grid voltage through a phase locked loop (PLL) circuit, wherein a phase difference of a predetermined size is recognized as an occurrence of islanding. However, this method fails when the load power factor is one.
The voltage harmonic monitoring method uses a principle that, when a photovoltaic generation system is in an islanding state, the voltage harmonic component increases due to exciting current supplied to a distribution transformer. However, when a non-linear load exists, setting an appropriate harmonic value to disconnect an inverter is nearly impossible.
These passive methods have a defect that there exists a non-detection zone (NDZ). The passive detection methods have a problem in that detecting an islanding operation is impossible in a specific situation, for example, in a case where a voltage, a frequency, or a change therein is very small.
Methods of introducing a predetermined change portion into the inverter output in order to remove the non-detection zone are called active methods.
In the output power variation method, the output voltage is continuously observed while the effective power of the inverter is being periodically perturbed. When islanding occurs, voltage fluctuations become large due to effective power mismatching, which makes it possible to stop the system. However, this method is ineffective due to the averaging effect.
The active frequency drift (AFD) method is to observe the frequency of a terminal voltage while minutely increasing or decreasing the frequency of the output current of the inverter. The frequency of the terminal voltage exceeding a predetermined range indicates islanding. However, this method has a defect that it is easy to fail when the phase angle of a load is identical to a phase value generated by a changed portion in the frequency.
The sliding mode frequency shift (SMS) method is applied in a similar manner as that of the AFD method. In this method, the starting angle of inverter current is controlled, and a change in the frequency generated under islanding is observed. This method also fails in detecting an islanding operation when the phase of load current is equal to the starting angle of the output current of the inverter.
The active methods have an advantage in that it is possible to reduce the non-detection zone caused in the passive methods, but has a disadvantage in that forcibly introducing a minute change signal into the output of an inverter degrades the quality of power and reduces the output efficiency of the inverter.